1. Field of the Invention
The present invention relates to focus detection devices having an arrangement of filters, e.g., infrared cut filters, for use in cameras and the like.
2. Description of Related Art
Generally, in focus detection devices that utilize phase difference methods (in which defocus amounts are calculated and then used to drive a lens), luminous flux is transmitted through different regions of the photographic lens and forms an image through a plurality of re-imaging lenses onto a corresponding plurality of line sensors. The focus state (e.g., a defocus amount) of the photographic lens is detected from the output of these line sensors. Photoelectric conversion elements, such as for example CCDs, which typically are utilized as these line sensors, generally have a peak spectral sensitivity in the infrared regions. Accordingly, a filter is used with the focus detection optical system in order to reduce light in the infrared regions. Such filters are referred to as infrared cut filters or infrared reduction filters. These filters change the luminosity factor of the photoelectric conversion elements, so that it is more comparable to the visual sensitivity of the human eye. Frequently, a reflecting type of filter has been utilized as a type of infrared cut filter. Such reflecting type filters are made by coating a glass substrate with many layers of film.
In general, the filter can be positionally arranged at a number of locations within the camera. However, there are many cases in which the view field mask, in the vicinity of the primary focal plane of the photographic lens, is positionally arranged on the front plane of the field lens. This results in the infrared cut filter being positioned on the front side of the field lens.
FIG. 2 shows an example of a positional arrangement of an infrared cut filter. FIG. 2 shows a focus detection device capable of focus detection in a plurality of areas. Specifically, the focus detection areas are provided at the center of the image plane and at two points, one to the left and one to the right of the image plane center. Luminous flux penetrating through the photographic lens L, passes through a half-transparent mirror M1 and is reflected by a reflecting mirror M2, so that the light rays reach a view field mask SM. Openings SM1-SM3 are provided in the view field mask SM at the prescribed focus detection areas. The opening SM1 corresponds to the focus detection area in the center of the image plane, and the openings SM2 and SM3 correspond to the areas at the left and right of the center of the image plane. The standard construction of the focus detection device generally includes a field lens L0, a reflecting mirror M3, an aperture mask AM, a re-imaging lens L2, and a line sensor CS, etc. As these elements are known in the art, a further explanation is omitted herein.
Typically, the function of the view field mask SM is to prescribe the focus detection areas. Therefore, in the case of a camera, the view field mask is positionally arranged in the vicinity of the primary focal plane, which is arranged in a position corresponding with the surface of the film. The infrared cut filter IR is arranged on the top side (in FIG. 2) of the view field mask SM. This is also referred to as the front surface of the view field mask SM. Additionally, in conjunction with the view field mask SM and the field lens L0, etc., the infrared cut filter IR is fixed by an adhesive or the like in a holder (not shown) for the focus detection device. The front surface (also referred to as the input surface or upstream surface) of the field lens LO has the smallest angle at which the luminous flux can enter, within the focus detection optical system. Accordingly, the infrared cut filter IR preferably is arranged on the front surface of the field lens L0, so that changes in the angles of the entering rays, which occur due to the change of transparency characteristic of the infrared cut filter IR, can be minimized. Additionally, as this filter includes a glass substrate, the entrance of dust or the like to the field lens LO is prevented by this location of the infrared cut filter IR.
It also has been proposed, in the event that the subject is dark or the contrast is low, that focus detection be performed by illuminating the subject. In such a case, auxiliary illumination light is cast from a direction other than that of the optical axis of the photographic lens from an auxiliary illumination device provided within the camera body, or within the accessories attached to the camera. Japanese Laid-Open Patent Application No. 63-82407 discloses a focus detection device capable of focus detection in three focus detection areas within the image plane, similar to what is shown in FIG. 2. An auxiliary illumination device is provided to illuminate each focus detection area, respectively.
FIG. 3 shows the construction of the optical system for such an auxiliary illumination device. FIG. 4 shows the relation of the auxiliary illumination luminous flux to the three focus detection areas. In order to illuminate the three areas, three auxiliary illumination light sources LD1-LD3, formed by LEDs or the like, are provided behind (on the right side in FIG. 3) a light-casting lens TL. In FIG. 4, p, q and r denote the auxiliary illumination luminous flux of light sources LD2, LD1 and LD3, respectively. The relation of the luminous flux to the focus detection areas is altered by the focal length of the photographic lens. When the photographic lens has a given focal length, the focus detection areas are defined respectively as P, Q and R. When the photographic lens has half that given focal length, the focus detection areas are defined as P2, Q2 and R2. In the event that the focal length is shorter than one half the given focal length, the areas to the left and right of the center area fall outside of the auxiliary illumination luminous fluxes p and r. Consequently, focus detection cannot be performed by utilizing auxiliary illumination light in the left and right areas for all desired focal lengths.
The device of Japanese Laid-Open Patent Application No. 63-82407 can operate in a mode in which first, the auxiliary illumination light sources LD1-LD3 all cast light. Then, the area (from amongst the three is areas) in which the reflective light quantity is the largest, is considered to be the area containing the nearest photographic subject. Accordingly, subsequent light casting for focus detection is performed only for that one area. Using this method, if the photographer selects the area containing the nearest subject as the area to focus upon, focus detection can be performed. One drawback of this process is that auxiliary illumination light must be cast two times. Thus, it takes a certain amount of time until photography can be performed, which is a drawback. In addition, the first light casting is performed simultaneously for all areas, resulting in a large consumption of power from the power source, which is a problem.
In view of the above problems, one might consider limiting focus detection area selection only to a manual selection by the photographer. In this case, preferably the auxiliary illuminating light is cast only on the manually selected area. Therefore, the light casting of illumination light need only be performed once, which avoids the problems described above. However, several auxiliary illumination light sources must be provided (e.g., one for each area) and it is necessary to provide space for them within the camera body or within the accessories, so that the camera becomes large in size. This arrangement still suffers from the problem, described above, when the focal length of the photographic lens becomes short, i.e., the left and right areas move out of the light casting range of the auxiliary illumination light. Consequently, focus detection cannot be performed by auxiliary illumination light within the left and right areas for all lens positions, which is a problem. The center area, however, is within the center of the light casting range regardless of the focal length of the photographic lens, and thus it is never outside of the light casting range.
Accordingly, as a method to allow there to be no restriction of the focal length of the photographic lens and so as to lessen the influence of the light casting of auxiliary illumination light on the size of the camera and the like, or on power source consumption, so that, for example, focus detection can be performed in several areas, even in areas in which there is a common level of brightness, one might consider providing an auxiliary illumination light source only for the center area, so that focus detection is performed only in the center area using auxiliary illumination light.
However, when this is performed in focus detection devices in which focus detection can be performed in the three areas denoted in FIG. 2, for example, the following type of problems occur. Generally, light from the LED or the like, which is used as an auxiliary illumination light source, has wavelengths that are on the longer side of the human visual region. However, as described above, the region in which the wavelengths are long exerts a negative influence on the focus detection accuracy due to the spectral sensitivities of CCDs. Thus, as described above, an infrared cut filter is provided. The light from the auxiliary illumination light also passes through the focus detection optical system, resulting in the wavelengths of this light being reduced by the infrared cut filter (so that light from the long wavelengths does not pass therethrough). In improving the accuracy of this focus detection system, the infrared cut filter blocks light having wavelengths above, for example, 680 nm. However, because the wavelengths of light from the LED are about 700 nm, if such an infrared cut filter is used, the auxiliary illumination light is also blocked. Therefore, the infrared cut filter used in focus detection optical systems that can use auxiliary illumination light is typically designed so as to block wavelengths above about 710 nm. In this case, as compared to a case in which a 680 nm filter is used, a certain extent of light source error due to long wavelength light from various types of light, must be allowed in regular focus detection precision.
In the construction of FIG. 2, one infrared cut filter IR is arranged in front of the field lens L0. In this construction, when focus detection is performed using the auxiliary illumination light in the center area, it is necessary to define filter IR as a filter that blocks light above a wavelength of about 710 nm. If this is done, then the cutoff wavelength of the right and left areas, in which focus detection using auxiliary illumination light is not performed, also becomes 710 nm, and the focus detection accuracy is detrimentally affected by long wavelength light from illumination during regular focus detection. Since focus detection accuracy in areas separated from the optical axis (i.e., at areas other than the center area) is influenced, for example, by aberrations and the like of the photographic lens, as compared to the center area, the accuracy on both sides (i.e., the left and right sides) drops. Further, with the same 710 nm filter, even though accuracy in the center area remains within a range in which there are no practical problems, accuracy drops in the left and right areas, which is a problem.
A filter exchange mechanism can be provided to change the filters during focus detection, and during auxiliary illumination light focus detection without causing any optically-related problems. However, this requires that a fairly complicated mechanism be provided around the focus detection device, which enlarges the camera body. Moreover, a problem occurs in that the filter exchange causes a long amount of time to be taken during photography, and the like. Furthermore, in a focus detection device performing focus detection in a plurality of areas, the device itself is already fairly large, and there is little available space for the introduction of a filter change-over mechanism.